Despite intensive efforts by investigators in major laboratories, the Norwalk and related caliciviruses have yet to be cultivated in any tissue culture system. This, along with the recent expression of 27nm virus-like particles of some members of this group of agents, as well as studies of expressed viral proteins of rotavirus, have contributed to the continuing important role of the electron microscope in the research program of the Epidemiology Section. The electron microscope continues to be the only method (i) for direct detection of the Norwalk and related viruses and derivative 27nm expressed virus-like particles and (ii) for unraveling the antigenic relationships among these fastidious agents. The electron microscope was used in the past year in individual experiments as an adjunct to other studies including examination of (i) preparations of expressed Snow Mountain virus-like particles for suitability for immunization; (ii) preparations from studies aimed at expressing Hawaii virus-like particles; (iii) preparations from bioreactor studies attempting to cultivate Norwalk virus; (iv) stool suspensions from individuals who developed illness following administration of candidate vaccine Wa x (DS-1 x UK), for the presence of viruses; (v) preparations of stools from volunteers who had been challenged previously with the Wollan (W) or Montgomery County (MC) agent to determine if virus particles were present; and (vi) preparations of mink calicivirus.